Electrothermal atomizers, commonly referred to as heated graphite atomizers or graphite furnaces, are utilized in atomic absorption spectrophotometers for rendering the sample to be analyzed in atomic form. Typically, the furnaces comprise a tubular graphite member clamped between annular graphite contacts, or electrodes, engaging its respective ends. A radial aperture in the side wall of the tubular member at the mid-point of its length serves as a sample port, accommodating the insertion into the tubular member of the substance to be analyzed.
The contacts, usually mounted in cooling jackets, are pressed into tight engagement with the ends of the tubular furnace member by resilient biasing means or a servomotor. An intense electrical current, passed longitudinally through the tubular member between the contacts heats the member to the high temperature required to convert the sample to a "cloud of atoms."
A beam of essentially monochromatic radiation having a wavelength corresponding to a characteristic spectral line of the substance (the "analyte") sought to be determined in the sample undergoing analysis, is passed longitudinally through the tubular member as permitted by the annular configuration of the electrical contacts. The atomic cloud absorbs the radiation in proportion to the concentration of the analyte in the sample; the reduction in intensity of the beam caused by the absorption is determined by a suitable detector and converted into an electrical signal.
In order to prevent rapid deterioration of the tubular graphite member by oxidation at the high temperatures required for atomization of the analyte, provision is made for enveloping it in a flow of inert protective gas.
Due to the heat sink effect of the contacts and their associated cooling jackets, the ends of the tubular member are cooler than the medial portion. This non-uniformity of temperature results in the deposition of sample on the cooler ends of the tubular member; the deposit is re-evaporated in subsequent use of the tubular member, contaminating the new sample.
A graphite furnace of the type described above is shown in U.S. Pat. No. 4,022,530. In this particular furnace, the electrode members are tubular, rather than annular, and coaxially envelopes the tubular graphite furnace member over substantially its entire length, except for a gap between the confronting ends of the electrodes. One of the tubular electrodes extends beyond the mid-point of the furnace member and contains a sidewall aperture registering with, and providing access to, the sample port.
Inert gas is introduced into the ends of the tubular member and flows outwardly through the sample port and the aperture in the tubular electrode aligned therewith.
In one attempt to achieve a more uniform temperature distribution in the tubular member of graphite furnaces it has been proposed to pass the heating current transversely through the furnace member rather than longitudinally. To this end, a contact arrangement is described in U.S. Pat. No. 4,407,582 employing two pairs of interconnected contacts in the form of bifurcated electrode members radially engaging a graphite tube on opposite sides. The heating current flows in the area of the ends in a circumferential direction through the graphite tube, heating it in the area of its ends. Heat flows from the ends to the center, achieving a more uniform temperature distribution.
In this known contact arrangement, the electrodes engage the hot parts of the graphite tube; consequently, the reproducibility of the contact characteristics is poor. Furthermore, it is difficult to protect the graphite tube from exposure to atmospheric oxygen by means of an inert protective gas flow, resulting in short useful life of the graphite tube.
In German Offenlegungsschrift 35 34 017 and the publication in "Analytical Chemistry", 58 (1986), 1973, there is described a graphite furnace in which the tubular furnace member is rectangular in cross-section and has integral contact pieces extending transversely to the axis of the tubular member. Thus, contact is effected in a cold area at plane surfaces. While this contact arrangement avoids the problem of non-reproducible contact characteristics, the difficulty of protecting the furnace from the entrance of air by an inert gas mantle remains.
Consequently, the furnace tube deteriorates rapidly in use and is, therefore, considered an expendable part. As the furnace with its integral contact is a relatively large graphite body, it is difficult to fabricate and, therefore, expensive. The combination of rapid deterioration and high replacement costs leads to a high cost per analysis.
It is the general object of this invention to overcome, or at least mitigate, the shortcomings of prior art graphite furnaces as outlined above.
It is a specific object of the invention to provide a contact arrangement of the above-described type having transverse heating current flow that lends itself to, and facilitates, effective protection of the furnace tube against exposure to atmospheric oxygen.
It is a further object of the invention to apply electric heating current to the furnace tube largely without loss.
Still another object of the invention is to cool the furnace well after each reading and thus to reduce the cooling time required prior to introduction of the next sample.
A further object is the provision of an electrothermal contact assembly in which a graphite component heated to high atomizing temperatures and consequently subject to rapid deterioration is relatively easy to fabricate and thus inexpensive.
To the accomplishment of the foregoing objects and others which will become apparent as this description proceeds, the invention contemplates an electrothermal furnace comprising a generally tubular furnace member having copolanar longitudinally extending contact ribs projecting radially outwardly from its outer surface on diametrically opposite sides. A pair of contact members having respective complementary mating surfaces disposed in confronting relation coact to define a cavity accommodating the furnace member and making electrical contact with the contact ribs of the furnace member when it is inserted in the cavity and the contact members disposed with mating surfaces in confronting relation.
As will be more fully appreciated as this description of exemplary embodiments of the invention proceeds, large contact surrounding the furnace member are provided enabling transmission thereto of a high intensity transverse heating current largely without loss. Enclosed in the cavity, the furnace member can be effectively protected from oxidation by an inert gas flow shielding the surfaces of the furnace from atmospheric oxygen. The configuration of the contact members enables large area surface contact with their cooling jackets inasmuch as this is not determined by integral projections of the furnace. In consequence of the large area surface contact, the furnace contact members and concomitantly the furnace can be rapidly cooled after each measurement. The furnace member itself is small and relatively simple in design and therefore easily and inexpensively fabricated.
Exemplary embodiments of the invention will now be described with continued reference to the following drawings in which like reference numerals denote like parts throughout the several views.